1. Field of the Invention
The present invention relates to a fixing apparatus for use in a printer, copier, facsimile machine, or the like which employs an electrophotographic system or an electrostatic recording system.
2. Related Background Art
Various image forming apparatuses are conventionally known. Of these image forming apparatuses, those based on an electrophotographic system have prevailed which exposes a photosensitive member to laser light and which then develops an image. Such an image forming apparatus has advantages such as a high image quality and high-speed operations and is widely used as, for example, an output apparatus such as a copier or a color laser beam printer.
The color image forming apparatus such as a copier or a color laser beam printer has been requested to achieve high productivity (the number of prints per unit time) using various sheets (recording materials) such as cardboards.
In order to increase the productivity of the image forming apparatus to which the electrophotographic system is applied, notably in connection with paper with a heavy basic weight, it is necessary to increase the fixing speed of a fixing apparatus. However, paper with a heavy basic weight absorbs much heat. Accordingly, such paper requires a significantly larger quantity of heat for fixation than thin paper. Thus, at present, when images are fixed to paper with a heavy basic weight, a fixing process is executed with a reduced fixing speed.
Thus, as a technique for enabling an increase in the speed at which color images are fixed, a fixing apparatus configuration has already been proposed which is disclosed in Japanese Patent Application Laid-Open Nos. H10-149044 and 2004-37555.
An image heating fixing apparatus according to Japanese Patent Application Laid-Open No. H10-149044 has paired fixing members disposed so as to be rotatable while in pressure contact with each other. An unfixed image is formed on a recording material at a nip portion formed by fixing member pair. While conveyed, the recording material is sandwiched between the fixing members to heat and fix the unfixed image. One or more external heating members can abut against and retract from the fixing member pair.
Further, Japanese Patent Application Laid-Open No. 2004-37555 discloses an external heating apparatus and a fixing apparatus using the external heating apparatus. The external heating apparatus comprises a plurality of external heating rolls that come into pressure contact with a surface of a heated member to externally heat the surface. The external heating apparatus has contacting and separating means for bringing the plurality of external heating rolls into pressure contact with the surface of the heated member and separating the external heating rolls from the surface and adjusting means for bringing the plurality of external heating rolls into uniform pressure contact with the surface of the heated member.
The above conventional techniques have the following problems. With the fixing apparatuses according to Japanese Patent Application Laid-Open No. H10-149044 and Japanese Patent Application Laid-Open No. 2004-37555, even if recording materials (referred to as transfer paper below) are cardboards, the following measure can be taken to realize a high productivity substantially equivalent to that for thin paper; the quantity of heat generated by the external heating rolls is increased to allow the external heating rolls to provide more heat to the surface of the heated member. However, with a mere increase in the quantity of heat provided by the external heating rolls, when the fixing apparatus fixes an image to transfer paper, if the external heating rolls provide a large quantity of heat to the surface of the heated member before the transfer paper enters a fixing portion, then the surface of the heated member becomes too hot. This causes toner on the transfer member to be excessively melted to make the toner too glossy. Thus, an attempt to control the external heating temperature or the temperature of the heated member cannot solve the above problems because of the insufficient thermal responsiveness of the external heating rollers or heated member.
Further, if an attempt is made to cause the external heating rolls to provide a quantity of heat to the surface of the heated member after the transfer paper has entered the fixing portion, the transfer paper absorbs heat from the heated member to make the heated member too cold. Consequently, the toner on the transfer paper cannot be melted and thus has a reduced gloss. As a result, the toner gloss becomes nonuniform (gloss nonuniformity). It is thus impossible to effectively heat the surface of a fixing roll that is the heated member, using the external heating rolls. This may in turn make it impossible to achieve a high productivity or vary the gloss of color images (gloss nonuniformity). Therefore, disadvantageously, high-quality color images are not obtained.
To solve these problems, it is possible to use the external heating rolls to provide a quantity of heat to the surface of the heated member the instant the transfer paper enters the fixing portion. However, in view of the deviation of timing of the abutment of the external heating rolls against the heated member and the deviation of timing of the entry of the transfer paper into the fixing portion, it is impossible to match the timing of entry of the transfer paper into the fixing portion with the timing of provision, by the external heating rolls, of a quantity of heat to the surface of the heated member. For example, it is assumed that an attempt is made to increase the productivity of a copier and that the transfer paper is conveyed at a speed of 300 mm/sec in order to achieve a productivity of 70 CPM. When the deviation of timing of the abutment of the external heating rolls against the heated member is 100 msec in view of the responsiveness of a motor or the like and the deviation of timing of the entry of the transfer paper into the fixing portion is 200 msec in view of conveying speed errors, the resulting timing deviation is about 300 msec. In terms of distance, this corresponds to a deviation of 90 mm.